J. Cao et al Materials and Design 32(2011)2763-2770 to non-spontaneous nucleation theory of welding crystallography Acknowledgements [26-28]. firstly, melted filler material crystallizes on the base of djacent base material, and then produces epitaxial growth to form The work was supported by both National Natural Science coarse columnar crystals under the effect of temperature field. Foundation of China(Grant 50871076)and Shanghai Leading Aca- Thus, the direction where the grains in weld metal zone grow up demic Discipline Project(Project Number: B113) decided by variation of temperature field. Taking into account lat the direction where temperature decreases is from the inter- References face between base material and weld metal towards the e middle part of the weld metal zone, the melted weld metal crystallizes [1 Ennis P). zielinska-lipiec A, Wachter o, Czyrska-filemonowicz A. tion. In other words, the grains in weld metal zone grow up from (2 Kodak &. Hemas aw Kielbus at s uastrcture stability of h the two interface sides towards the middle part of weld metal zon respectively and result in orientation distribution along the trans- [3I Kimura K, Sawada K Effect of stress on the creep deformation of ASME Grade transverse tensile strength of weld metal is improved for its orien- 1492 anew. ande nberghe B. Hahn B, Heuser H. Jochum C T/P23.24,911 and erse direction of the joint, seen in Figs. 8 and 10. Therefore, the tation in this direction. While in the two HAZs, because of narrow I5)Haarmann K vaillant JC, Vandenberghe B, Bendick W, Arbab A. The T91/p91 perature gradient change. This suggested that orientation distribu- [6] Richardot D, Vaillant JC, Arbab A, Bendick w. The T9 tions of grains in the two HAZs are random. On the other hand, for T92 HAZ, since the temperature is higher than its initial austenitic transition temperature(Ac, during the welding process, tempered martensite begins transforming into [81 Abe F, Horiuchi T, Taneike M, Sawada K. Stabilization of martensitic ustenite. Furthermore, in t92 CGHaZ, the temperature even ex- are in advanced 9Cr steel during creep at high temperature. ceeds its fully austenitic transition temperature (Ac3), which [91 makes austenitic grains grow up and results in coarsening. while in T92 FGHAZ, as the temperature is between Aci and Ac3. the fine 101 Hu ZF, Yang zG. An investigation of the embrittlement inX20CrMov121power austenitic grains can be obtained. After welding, with the continu exposure at elevated te ous cooling, austenitic grains transform into lath-shaped martens- 11 Hu ZF, Yang zG. Identification of the precipitates by TEM and EDs in ite. Finally, t92 CGHAZ has martensitic structure with coarse grain ong-term service at elevated temperature. J. Mater En 2003:12(1):106-11 size and t92 FGHAZ has martensitic structure with fine grain size. [121 Serrea l, Vogt JB. Mechanic rties of a 316L/191 weld joint tested in 30(9):3776-83. austenite structure. Thus, microstructural evolution leads to the [14] Sawaragi Y, Otsuka N. of a new18-8 austenitic steel tube (Super304H) for fossil fired boilers after service exposure with high elevated variation of mechanical properties of the joints. [15 Dischi A, Kenny JM, Mecozzi MG. Development of high nitrogen low nickel aragi Y. Properties and experiences of a new 18-8 austenitic 4. Conclusions stainless steel tube(0. 1C-18Cr-9Ni-3Cu-Nb, N) for boiler tube application. The investigations performed on the T92/S304H dissimilar [171 Yasutaka N, Mitsuo M, Hirokazu O, Masaaki L Effect of grain size on creep fatigue properties of 18Cr-9Ni-3Cu-Nb-N steel under uniaxial and torsional materials joints have led to the following conclusions [18] ASTM E8-04. Standard test methods for tension testing of metallic materials. materials joints obtained by GTAW process can meet the USC 19 ASTM E23-02a. Standard test methods for notched bar impact testing of Tensile strength and impact toughness of T92 /S304H dissimilar boiler's requirements. Furthermore the part of the joints with [201 Maruyama k, Sawada k Koile J Strengthening mechanisms of creep resistant relatively weak tensile strength was T92 CGHAZ, while the part (21) Huang YZ, Titchmarsh JM. TEM investiga intergranular stress corrosion cracking for 316 stainless steel in PwR environment. Acta Mater weld metal 2. The coarse tempered martensitic structure of T92 CGHAZ makes [22] Kashyap KT, Chandrashekar T. Effects and mechanisms of grain refinement in the strengthening effect caused by grain boundary decreased [23] Sleboda T. Muszka K, Majta I. Hale P. Wright RN. The possibilities of and results in relatively weak tensile strength in T92 CGHAZ echanical property control in fine grained structures. J Mater Process Tech 3. The weak toughness of weld metal is attributed to its 2006;177(1-3)461-4. dendritic austenitic structure. Furthermore, the impact [24] Herring DH. Grain size and its influence on materials properties. Heat Treat graph of weld metal takes on typical brittle re [25] He Z]. Mechanical properties of metal materials. 1st ed. Beijing: Metallurgical characteristics 4. Grains in weld metal zone grow up respectively from the two 126 Ne son W, Ltppoid j c.mis My.nature and evol ution ot the tusion bounday interface sides towards the middle part of weld metal resulting in its orientation distribution in transverse direction of the [27] Savage wF, Lundin CD, Aronson AH. Weld metal solidification med joints.Weld metal. due to its orientation distribution, has v)Yyhermoch Acta 1996: 280: 303-17 ystallization kinetics of amorphous alloys. higher tensile strength than T92 CGHAZto non-spontaneous nucleation theory of welding crystallography [26–28], firstly, melted filler material crystallizes on the base of adjacent base material, and then produces epitaxial growth to form coarse columnar crystals under the effect of temperature field. Thus, the direction where the grains in weld metal zone grow up is decided by variation of temperature field. Taking into account that the direction where temperature decreases is from the inter￾face between base material and weld metal towards the e middle part of the weld metal zone, the melted weld metal crystallizes along this direction, i.e. the maximum temperature gradient direc￾tion. In other words, the grains in weld metal zone grow up from the two interface sides towards the middle part of weld metal zone respectively and result in orientation distribution along the trans￾verse direction of the joint, seen in Figs. 8 and 10. Therefore, the transverse tensile strength of weld metal is improved for its orien￾tation in this direction. While in the two HAZs, because of narrow width in transverse direction of the joints, there is no obvious tem￾perature gradient change. This suggested that orientation distribu￾tions of grains in the two HAZs are random. On the other hand, for T92 HAZ, since the temperature is higher than its initial austenitic transition temperature (Ac1) during the welding process, tempered martensite begins transforming into austenite. Furthermore, in T92 CGHAZ, the temperature even ex￾ceeds its fully austenitic transition temperature (Ac3), which makes austenitic grains grow up and results in coarsening. While in T92 FGHAZ, as the temperature is between Ac1 and Ac3, the fine austenitic grains can be obtained. After welding, with the continu￾ous cooling, austenitic grains transform into lath-shaped martens￾ite. Finally, T92 CGHAZ has martensitic structure with coarse grain size and T92 FGHAZ has martensitic structure with fine grain size. However, For S304H austenitic steel, there is no martensite– austenite (M–A) transition during welding process. Only the region adjacent to the weld metal, due to welding heat effect, has coarse austenite structure. Thus, microstructural evolution leads to the variation of mechanical properties of the joints. 4. Conclusions The investigations performed on the T92/S304H dissimilar materials joints have led to the following conclusions. 1. Tensile strength and impact toughness of T92/S304H dissimilar materials joints obtained by GTAW process can meet the USC boiler’s requirements. Furthermore, the part of the joints with relatively weak tensile strength was T92 CGHAZ, while the part of the joints which revealed relatively weak toughness was weld metal. 2. The coarse tempered martensitic structure of T92 CGHAZ makes the strengthening effect caused by grain boundary decreased and results in relatively weak tensile strength in T92 CGHAZ. 3. The weak toughness of weld metal is attributed to its coarse dendritic austenitic structure. Furthermore, the impact fracto￾graph of weld metal takes on typical brittle fracture characteristics. 4. Grains in weld metal zone grow up respectively from the two interface sides towards the middle part of weld metal resulting in its orientation distribution in transverse direction of the joints. Weld metal, due to its orientation distribution, has higher tensile strength than T92 CGHAZ. 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